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Abstract

White light emitting diode (LED) systems, capable of lowering the color temperature of emitted light on dimming, have been reported in the literature. These systems all use multiple color LEDs and complex control circuitry. Here we present a novel responsive lighting system based on a single white light emitting LED and a thermoresponsive scattering coating. The coated LED automatically emits light of lower correlated color temperature (CCT) when the power is reduced. We also present results on the use of multiple phosphors in the white light LED allowing for the emission of warm white light in the range between 2900 K and 4150 K, and with a chromaticity complying with the ANSI standards (C78.377). This responsive warm white light LED-system with close-to-ideal emission characteristics is highly interesting for the lighting industry.

Figures (12)

Schematic depiction of the LED-package with the thermo-responsive coating
on top. The picture on the left shows the device at low
temperature/current, where the coating is in a scattering state. In the
picture on the right the current/temperature is high and the coating is
thus transparent. The color of the coating in this schematic depiction
shows the color of the light leaving the device. For the sake of clarity
the coating is depicted out of contact with the LED package; it should
be noted, however, that the coating is brought into optical and thermal
contact with the LED package.

a) Correlated color temperature (CCT) of the light emitted by the LED at
different currents and temperatures applied to a cold white light LED
coated with the thermo-responsive coating. The black diamond symbols
show the uncoated LEDs. The red squares display the first run of
increasing (closed symbols) and decreasing (open symbols) the current.
The red circles display the second run. b) Demonstrator consisting of a
row of coated cold white LEDs on the left-hand side and a row of bare
cold white LEDs on the right-hand side. The top picture shows the
demonstrator operating at a low current (~20 mA/LED) and the bottom
picture at a high current (~60 mA/LED). In order to capture the
switching of CCT on camera a diffuser is placed on top of the
demonstrator at a large distance (approximately 5 cm).

The CIE 1931 diagram depicting the chromaticities of the light emitted by
an uncoated cold white light LED (black data points) and a coated cold
white light LED (blue data points), measured at different currents (5-60
mA). The right figure is an expansion of the relevant part of the left
figure. The purple line represents the connection between the value in
color space of the blue LED and the phosphor in question. The black
curve represents the ideal black body emitter. The ellipses depict
5-steps MacAdam ellipses around black body emitters at a certain
temperature.

a) Correlated color temperature (CCT) of the light emitted by the LEDs at
different currents and temperatures. The black symbols display the
uncoated LED and the red symbols the coated LED. The red squares depict
the first run of increasing (closed symbols) and decreasing (open
symbols) the current. The circles display the second run. b)
Demonstrator consisting of a row of coated warm white LEDs (bottom) and
a row of bare warm white LEDs (top). The top picture shows the
demonstrator operating at a low current (~20 mA/LED) and the bottom
picture at a high current (~80 mA/LED). In order to capture the
switching of CCT on camera a diffuser is placed on top of the
demonstrator at a large distance (approximately 5 cm).

CIE 1931 diagram depicting the chromaticities of the light emitted by an
uncoated (black) and a coated LED (blue), measured at different currents
(5-90 mA). The right figure is an expansion of the relevant part of the
left figure. The red and yellow lines represent the connections between
the chromaticity of the blue LED and the two different phosphors. The
black curve represents the ideal black body emitter. The ellipses depict
5-steps MacAdam ellipses around black body emitters at a certain
temperature. Several CCT lines are shown as black dotted lines.

CIE 1931 diagram depicting the chromaticities of the light
emitted by various samples of coated warm white LEDs
measured at different electrical currents (5-60 mA). The
data points shown in different colors were obtained by
polymerization at different temperatures. All coatings
contained 50% of compound 3. The black curve
represents the ideal black body emitter. The ellipses depict
5-steps MacAdam ellipses around black body emitters at a
certain temperature. Several CCT lines are shown as black
dotted lines.